As neat as adding boats and rafts to the main game would be, I’m gonna have to echo the others saying they just wouldn’t be terribly practical on this map, on account of they’d be basically useless outside of the big river. Now a new map, on the other hand…
Funny story, I’ve actually found recipes for what seem to be pontoon foundations in the game, which sounds an awful lot to me like a floating platform of some type. You can’t actually craft them though, so either they’re something planned for the future but not fleshed out yet or an idea that was abandoned in the past for whatever reason.